The invention relates to solution polymerization of conjugated dienes and vinyl aromatic compounds. The invention further relates to random copolymers of the conjugated dienes and vinyl aromatic compounds in which more than 60% of the monomer units are arranged in an arbitrary manner.
Many processes for the preparation of substantially random copolymers of conjugated dienes and vinyl aromatic compounds are well known in the art. Typically, the solvents described for the known polymerization processes include essentially any aliphatic, cycloaliphatic or aromatic compound or a mixture of these compounds, provided that the compound is essentially inert. Cyclohexane alone or in combination with another compound is a highly preferred solvent in prior art processes.
Of all random polymerization processes, the one as disclosed in British patent specification No. 1,283,327, is particularly useful. First, a starting mixture is prepared from the solvent and part of the totally needed quantity of each of the monomers, subsequently the copolymerization is initiated by contacting this mixture with the initiator, and during copolymerization the monomer ratio in the reaction mixture is kept constant by addition of the remaining part of each of the monomers.
The random copolymerization processes referred to above are suitably carried out at temperatures in the range of from 50xc2x0 C. Often compounds having a lower boiling point than cyclohexane, i.e. lower than 80xc2x0 C., are added to the cyclohexane containing polymerization medium, in such concentrations that the solvency power is still sufficient, in order to provide enough vapor pressure to be able to remove the heat of polymerization by means of evaporative cooling.
The invention is a process for preparing a random copolymer of a conjugated diene and at least 50% by weight of a vinyl aromatic compound in a non-polar solvent with the aid of a chelating modifier to achieve at least 50% 1,2-addition of the conjugated diene. The monomers are polymerized with a lithium initiator in the presence of a chelating modifier such as ethylene glycol diethyl ether. The reaction proceeds slowly with continuous addition of monomers at 10xc2x0 C. to 40xc2x0 C. to give random polymerization. Control of the polymerization reaction is enhanced by adding the chelating modifier after initial polymerization of at least 5% of the monomers.
As a result of extensive research and experimentation a process has now been found in which the random polymerization of conjugated dienes and vinyl aromatic compounds is achieved in a non-polar solvent which contains a chelating modifier such as ethylene glycol diethyl ether to make random copolymers having at least 50% 1,2-addition of the conjugated diene. The random polymers have at least 50% by weight of the vinyl aromatic compound which is evenly distributed over most of the polymer molecule.
In the process according to the present invention polymerization rate constants for the monomers in the non-polar solvents containing the chelating modifiers are maintained significantly higher than the rate of adding monomers, which results in more random distribution of the 1,2 polymerized conjugated diene throughout the polymer molecules.
Since the polymerization process according to the present invention is carried out at low temperatures from 10xc2x0 C. to 40xc2x0 C., the danger of thermal decomposition of the living polymer chain end is low.
Chelating modifiers which will result in at least 50% 1,2-polymerization in non-polar solvents include ethylene glycol diethyl ether, propylene glycol diethyl ether, or tetramethyl-ethylenediamine, preferably ethylene glycol diethyl ether. The chelating modifier is present in the non-polar solvent, preferably cyclohexane, at a concentration from 500 ppm to 10,000 ppm, preferably 1,000 ppm to 4,000 ppm.
In order to control molecular weight of the random polymers, the totally needed quantity of initiator is added to a starting mixture of monomers under homogenization in a comparatively short time. The amount of monomers contained in the starting mixture is from 1% to 10% by weight of the total amount of monomers to be reacted. The chelating modifier is added after formation of small living polymers since earlier combination of the chelating modifier and the initiator results in an uncontrolled reaction between the chelating modifier and the initiator. The loss of significant amounts of the initiator results in an uncontrolled increase in molecular weight and the loss of significant amounts of the chelating modifier results in an uncontrolled reduction in 1,2 addition.
The copolymerization reaction is preferably terminated by means of substances which kill the living polymer; this can be a proton releasing compound, for instance water, an alcohol, an amine or protonic acid, a coupling agent, or a functionalizing agent such as carbon dioxide or ethylene oxide.
The aromatic vinyl compound is preferably styrene, but may consist of another mono-vinyl aromatic compound for example: 1-vinylnaphthalene, 3,5-diethylstyrene, 4-n-propylstyrene, 2,4,6-trimethylstyrene, 4-phenylstyrene, 4-methylstyrene, 3,5-diphenylstyrene, 3-ethyl-1-vinylnaphthalene 8-phenyl-1-vinylnaphthalene or a mixture thereof or mixtures containing predominantly styrene.
The conjugated diene is one capable of copolymerization with styrene or another aromatic vinyl compound and such that when polymerized with styrene or another selected aromatic vinyl compound or compounds, it provides a polymer having the desired properties. The diene is preferably 1,3-butadiene, but may be another diene, for example, 1,3-pentadiene, 2-methyl-1,3-butadiene (isoprene), 2,3-dimethyl-1,3-butadiene, 2-methyl-1,3-pentadiene or 2,3-dimethyl-1,3-pentadiene or mixtures of them alone or with butadiene.
After polymerization of the starting mixture, monomer addition preferably occurs by continuous addition of a premixed batch of the monomers to achieve the best distribution of the monomers. Although not preferred, periodic addition of separate portions of each monomer can be controlled to give good distribution of the monomers. As a result of the steps mentioned, the monomer concentration in the reactor is kept substantially constant which facilitates the control of the process.
Also special preference is given to the addition of the monomers during the copolymerization at a rate less than the rate at which the concerning monomer is consumed. Under these conditions the relative rate at which both monomers are applied during the copolymerization can be kept substantially constant at a value calculated beforehand and the heat development can be calculated in advance to control the temperature of the polymerization.
The lithium initiator is an alkyllithium compound, such as methylenedilithium, isopropyllithium, n-butyllithium, sec-butyllithium, amyllithium, 2-ethylhexyllithium, phenyllithium, ethylenedilithium, trimethylenedilithium, pentamethylenedilithium, 1,4-dilithiobenzene, 1,5-dilithiobenzene, 1,5-dilithionaphthalene and 1,3,5-trilithiumpentane. The amount of initiator used in the process according to the present invention may vary within the wide limits to control molecular weight of the random copolymers.
Generally at the end of the reaction the copolymer containing the reaction mixture is pumped to a polymer recovery area. The principal step in recovery of the polymer comprises coagulation and eventual drying of the polymer to produce a crumb. Thus the cement may be coagulated by treatment with steam and/or hot water. Alternatively, the cement may be sprayed into a hot water bath under such conditions that a crumb is formed. The solvent is removed as a vapor and may be recovered and recycled as desired. The resulting copolymer-water slurry is withdrawn and passed on to a dewatering screen where the water passes through the screen leaving the rubber crumb. This may be reslurried with cold water, drained and finally dried by known means.
The random copolymers are useful within a peak molecular weight range from 5,000 to 250,000, preferably from 25,000 to 50,000. The random copolymers of the invention can be used as a replacement for styrene-butadiene rubbers (SBR""s) in general, and are most useful in toner compositions for copy machines as taught in EP 477,577.